Actually I just forgot about "Arena," but I wouldn't have counted it anyway. Even aside from what CorporalCaptain says about its origins, the episode "Arena" is way, way different from the Brown story in any case. There are enough broad similarities in the premise -- warring human and alien abducted by superadvanced race and placed in artificial environment to battle one another to the death -- that Gene Coon was concerned he may have been influenced by Brown's story and thus felt it appropriate to buy the rights from Brown. And of course they have the same title. But the characters, the alien, the setting, the story mechanics, and the outcome are all completely different. Here's the original story for comparison.

What makes "The Slaver Weapon" distinct from "Arena" and "Tin Man" is that it doesn't just take some broad concepts from the original and build a Trek story out of them, but it retells "The Soft Weapon" beat for beat.